As semiconductor circuits are highly integrated, fabrication of semiconductor devices having a multi-layer wiring structure is being rapidly developed. For the semiconductor devices having the multi-layer wiring structure, a trench wiring for interconnecting components arranged in horizontal directions as well as a via hole wiring for interconnecting components stacked in a vertical direction should be formed. Employed as such a multi-layer wiring structure is a dual damascene structure. Recently, a highly electromigration resistant metal having a low resistance, e.g., copper, is used as a wiring material and an organic Low-k material capable of ensuring a low dielectric constant, e.g., a SILK (manufactured by The Dow Chemical Company), is employed as an interlayer insulating material.
FIGS. 6A to 6C illustrate an exemplary process of forming a shoulder portion of a via hole of a dual damascene structure by using a photoresist (PR) layer for via hole formation. Formed on, e.g., a wafer for a formation of the dual damascene structure are a silicon oxide film layer (SiO2 film layer) 101, a silicon nitride film layer (SiN film layer) 102, an organic Low-k film layer (e.g., SiLK film layer) 103, a SiN film layer 104 and a SiLk film layer, in which a Cu wiring layer (Cu layer) 105 of a lower layer circuit pattern is formed, as shown in FIG. 6C. Herein, the SiO2 film layer 101 and the silicon nitride film layer (SiN film layer 102) are formed to serve as a first hard mask layer and a second hard mask layer, respectively, for use in forming a via and a trench. The SiLK film layer 103 is formed as an interlayer insulating film layer and the SiN film layer 104 formed underneath the SiLK film layer 103 serves as a stopper for use in forming the via hole. Formed on top of the stack of such layers is a photoresist film layer (PR layer) 107 having a pattern for use in forming the via hole.
In case of forming the via hole in the SiLK film layer 103, by using a gas (e.g., a gaseous mixture of a CF4 gas, an Ar gas and an O2 gas) capable of etching the SiN film layer 102 and the SiLK film layer 103, the SiN film layer 102 is etched according to a via hole pattern 108 of the PR layer 107 shown in FIG. 6A and then the SiLK film layer 103 is overetched down to a predetermined depth, e.g., at least a depth corresponding to a shoulder portion of the dual damascene structure (see FIG. 6B). Thereafter, an etching (ashing) process is performed on the PR layer 107 by employing an etching gas (e.g., a gaseous mixture of a N2 gas and a H2 gas) having a high selectivity against both the SiO2 film layer 101 and the SiN film layer 102. During this process, the PR layer 107 is removed and, at the same time, the SiLK film layer 103 is etched by using the SiO2 film layer 101 and the SiN film layer 102 as a hard mask, thereby deepening the via hole 108.
However, in case of simultaneously etching the PR layer 107 for use in the formation of the via hole and the SiLK film layer 103 serving as the interlayer insulating film layer by using the gaseous mixture of N2 gas and H2 gas as in the prior art, there remain resist residues D on a trench portion 110 (on the SiN film layer 102) formed in etching the SiO2 film layer 101 as shown in FIG. 6C. This results in a poor etching profile of the SiN film layer 102 generated during a subsequent etching process thereof due to the resist residues, which in turn causes etching residues in a following etching process of the SiN film layer 103. Particularly, when an antireflection film layer is provided underneath the PR layer 107, a greater amount of resist residues tends to remain after the etching.